Ordinarily, an x-ray beam generating device, referred to as an x-ray tube, comprises dual electrodes of an electrical circuit in an evacuated chamber or tube. One of the electrodes is a thermionic emitter incorporated in a cathode assembly which is positioned in spaced relationship to a rotating disc-shaped target anode in the tube. Upon energization of the electrical circuit connecting the electrodes, the thermionic emitter is electrically heated to produce a supply of electrons which are appropriately accelerated and focused to a thin beam very high velocity electrons striking an annular section of the rotating disc anode. The annular section of the anode surface being struck by the electron beam comprises a surface of a predetermined material, a refractory metal for example, so that some of the kinetic energy of the striking electrons is converted to electromagnetic waves of very high frequency (x-rays). These electromagnetic waves proceed from the target anode, are collimated through an x-ray window in the surrounding tube wall and penetrate an object, such as human anatomic parts for medical examination and diagnostic procedures.
As is well known in x-ray practices, x-rays from the structure as described are caused to pass through the object to be examined and then impinge on an image detector, such as a solid state detector, a photographic film or plate, etc.. to provide an accurate visual representation of certain internal features of the object or anatomy. A high degree of resolution in the image obtained by this procedure is significant and necessary, particularly in medical practices for correct diagnosis. Consistent and improved image quality is influenced by a number of variables in x-ray tube design and operation. For example, the size of the focal spot of the impinging electron beam on the anode is a key contributor to x-ray image quality. From a number of x-ray image operations, it is noted that as the size of the focal spot increases, image resolution decreases. However, for a given x-ray tube electric power level, as the size of the focal spot decreases, the temperature of the impact region on the disc sharply increases, leading to decreased x-ray tube life expectancy. Accordingly, the design or selection of focal spot sizes is a compromise of required image quality and tube life.
It is an important object of this invention to provide an adjustable or variable focal spot size in an x-ray tube for improved image quality while maintaining desired tube life expectancy.